Polyester materials, particularly polyethylene terephthalate, exhibit a very high mechanical strength (compression/shear strength and modulus) and an excellent temperature resistance, but behave brittle, very often characterized by poor values of shear elongation at break and low impact strength. The brittleness of the polyesters confines its application, it often makes such applications impossible where for example a highly periodic mechanical loading occurs and/or polyesters need to be thermoformed to 3D articles and/or mechanical post-processes like screwing, nailing or bending are necessary.
Impact resistance is the ability of a material to resist breaking under a shock loading or the ability of a material to resist fracture under stress applied at high speed. Polyester materials, particularly aromatic polyethylene terephthalate are typically poor in impact resistance and break easily under shock loading. The free-fall impact testing of a foamed PET having a density of about 100 kg/m3 according to ASTM F1292, for instance, demonstrates relatively high values of g-max and HIC (Head Injury Criteria) (s. comparative example 1).
In a report of Throne (Throne, J. L., et al., Journal of Cellular Plastics, 21 (1985) 2, 123-140) the impact properties of structural foams of some thermoplastics were studied and the testings found no apparent correlation of impact properties with the foam density. Therefore, it is not expected that foamed polyesters with low density provide automatically a better impact strength.
The ductility and flexibility of an expanded polyester material are also limited: Currently, foamed polyester with a density of approximately 110 kg/m3 has a shear elongation at break of about 3% (according to ISO 1922), with compression strength values of 1.2-1.4N/mm2 (according to ISO 844).
A foamed polyester with a lower density such as approximately 100 kg/m3, as a further example, shows very often a shear elongation at break lower than 5%. At this density level, the compression strength is about 1.0N/mm2. On the contrary, some competitive materials show a much better ductility, e.g. foamed PVC with a density of 75-85 kg/m3 possesses a shear elongation at break of approximately 20% with compression strength of approximately 1.3N/mm2.
An expanded polyester with advanced ductility and impact resistance improves the thermoformability and imparts a better fatigue behaviour of end product. However, an improvement of material ductility often results in decrease of mechanical strength or/and rigidity of a thermoplastic polymer. Increase of ductility and flexibility while maintaining the mechanical strength/rigidity is a challenge in material science.
The overall objective of this invention is to develop and provide expanded polyester materials with a very high compressive strength and a high shear modulus, but with a minimum possible brittleness. This means that sufficient shear elongation is needed to target a widespread application of polyester materials, particularly used as a core material in a sandwich structure, where very particularly a dynamic loading occurs. Furthermore this can be used as an insulation material with excellent compatibility to construction materials, e.g. concrete, excellent mounting properties, e.g. for screwing or nailing. Such expanded materials should for instance be in position to resist a shortterm resin treatment up to 180° C. without any suffering from the mechanical strength afterwards, can be much better thermoformed to thick 3D articles, structurally integrated into building materials and exposed to periodic mechanical loading.
Elastomeric compounds are generally used as impact modifiers for polyesters in traditional thermoplastic processes. The effectiveness of the impact modification is highly dependent on:                the modifier type        the modifier content        the modifier particle size        the interparticle distance        
The elastomeric modifiers can be divided into non-reactive and reactive groups. The most non-reactive elastomeric modifiers such as general purpose rubbers are not highly effective at polyesters because they are unable to adequately interact with the polyester matrix. The poor interaction of the non-reactive modifiers with the polyester matrix is the main reason for the fact that optimally sized dispersed phases and strong interfacial bonding can not be achieved.
It is known from the literature (Sheirs, J., et al., Modern Polyesters, John Wiley & Sons Ltd (2003)) that the use of reactive compatibilization supports small dispersed elastomeric particles and a small interparticle distance to obtain a finely sized dispersed phase in polyester matrix, whereas reactive impact modifiers are grafted to polyester matrix. Reactive impact modifiers have functionalized end groups, which bond the impact modifier to the polymer matrix and moreover modify the interfacial energy between the polyester matrix and the impact modifier for enhanced dispersion.
Typical and commercially available reactive elastomers such as ethylene-ethyl acrylate-glycidyl methacrylate terpolymer (EEA-GMA), ethylene-butyl acrylate-glycidyl methacrylate terpolymer (EBA-GMA), ethylene-vinyl acetate-maleic anhydride (EVA-MA) and styrene-ethylene butylenes-styrene-maleic anhydride (SEBS-MA) have functionalized/reactive end groups of glycidyl methacrylate (GMA) or maleic anhydride (MA). These end groups are responsible for grafting of said elastomers to polyester matrix by a chemical reaction with carboxyl acid and hydroxyl end groups of the polyester resins.
Expanding of polyesters is nowadays more and more practiced by a reactive process comprising upgrading or increasing of molecular weight and extensional viscosity of aromatic polyester resins during the extrusion process with help of chain-extenders such as multifunctional tetracarboxylic dianhydrides.
The inventors of US 2003/0135015 A1 (Fujimaki, T.) mention in the description that, beside other thermoplastic materials, polyethylene acrylate resins, which are acrylic elastomers also acting as thermoplastic elastomeric modifiers, can be used as carrier material in the composition of a polyfunctional masterbatch. The epoxy-containing masterbatchs increase the molecular weight of polyesters and reduce the MFR. As a result, highly foamed materials can be produced. However, polyethylene acrylate resins apply as elastomeric modifiers in polyester foams were neither discussed in the description, nor supported by the examples.
The invention EP 2 048 188 A1 (Severini, T.) describes the use of acrylic elastomers as carrier material in a masterbatch formulation to increase the flexibility of foamed PET having a density around 130 kg/m3. An addition of such masterbatch containing PMDA (chain-extender) and acrylate leads to a better value of shear elongation at break (ranging from 17% to 25%). However, the application of this kind of masterbatch for an improvement of flexibility is limited in terms of composition constraint: An individual dosing of the acrylic elastomer for a requested value of material flexibility or for processing of different polyester grade is not possible because of a fixed need of PMDA in process. A higher or lower dosing of PMDA in the final composition of polyester process may results in a cell structure or foam properties which are not useful. As a result, a new masterbatch has to be developed for every individual application. Moreover, the influence of the acrylic elastomers on the stiffness of foamed polyesters is neither understood nor examined: The results of comparative and innovative examples are not comparable due to an absence of PMDA in the composition of the comparative examples. The better values of compression strength and shear modulus shown in the innovative examples are obviously resulted from chain-extending reaction of PMDA with PET during the foaming process. In addition, the impact resistance of PET foam produced with the help of the masterbatch containing the acrylic elastomers is neither evident nor of interest in said invention.
The acrylic elastomers are in general thermoplastic copolymers containing an acrylate content from 3 to 50% by weight and having a melt flow index from 0.1 g to 200 g/10 min. at 190° C./2.16 kg (according to ISO 1133). The acrylic elastomers can be divided into groups of 1) non-reactive acrylate copolymers and 2) reactive acrylate copolymers. Typical non-reactive copolymers are e.g. ethylene-ethyl acrylate copolymer (EEA), ethylene-butyl acrylate copolymer (EBA) and ethylene-methyl acrylate copolymer (EMA), while the reactive copolymers are cited above.
However, application of the reactive modifiers in a reactive expansion process is difficult, if not even impossible. Until now it was rarely reported for the characteristic reasons of such expanding process and limitation of such reactive modifiers. These reasons are described below:
1. It is acknowledged in the literature (Sheirs, J., et al., Modern Polyesters, John Wiley & Sons Ltd (2003)) that the amount of a reactive and/or non-reactive modifier required to achieve significant advantages from a mechanical point of view must range from 20 to 30% by weight of polymer matrix. During an expansion process like foam extrusion, a free expansion of the melt system containing physical blowing agents is applied, which is extremely sensitive to nucleation and melt strength. Any disturbance of nucleation and reduction of melt strength leads to formation of a poor and unacceptable cell structure during the free expansion. Addition of said modifiers in a high amount is thus not advantageous due to disturbance of nucleation and reduction of melt strength.
2. It is also perceived that reactive elastomeric modifiers exhibit high reactivity with polyesters, resulting in a chemical reaction with the functional end groups of polyester. This kind of reaction relates to lowering of the concentration of the functional groups of polyesters and causes an effect reduction of necessary polymer chain-extending enhancement, which is very essential for a reactive expanding process of polyesters.
3. Moreover, it is well-known that the use of elastomeric modifiers improves the flexibility, but worsens the modulus values of polyester products. Both properties are important for composite applications.